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Just kidding and a fair question. I believe that many if not all of the BMWs have the battery in the trunk area. I was told that the battery is under that small storage compartment in the rear center area of the trunk and you have to unscrew the two fasteners. If you check the engine compartment you will find two large metal studs for both the positive terminal (RED) and negative terminal for making external connection to the car. If I am in error someone jump in.

Conventional wisdom (for BMW) would put it in the trunk or under the rear seat. I looked in the trunk of my car and can see the fasteners jwr is talking about, given the size of the cover for the storage compartment and the much smaller available space under this cover it seems reasonable that this is where the battery is likely located.